There has been a gradual proliferation of undersea cables and pipelines connecting regions or countries separated by water. In particular the worldwide expansion of the telecommunications industry has been largely responsible for the increase in buried underwater cables.
Conventional cables are often buried beneath the seabed at a depth of approximately 1 meter, as this has traditionally provided protection from fishing methods and other environmental effects. The threat from fishing to conventional cable laying is mainly due to trawling “otter” boards which may penetrate the seabed up to a depth of up to 0.5 metres.
The maintenance or repair of such undersea cables is necessary for example to correct faults, repair damage or to replace such devices as fibre-optic cable repeaters.
One known method of recovering buried underwater cables is to drag a grapnel through the seabed from a surface ship. However, this method has a number of associated problems, particularly in that more than one cable may be hooked by the grapnel and retrieved at the same time, and the action of hooking a cable and raising it to the surface may cause the cable to be damaged or even break.
A more preferred method of recovering cables is to expose the cable by excavation of a wide shallow-sided depression in the seabed. This is generally performed using a surface vessel along with an underwater remotely operated vehicle (ROV).
A typical cable recovery operation involves the location of the relevant part of the cable followed by deploying an ROV to a position on the seabed close to the relevant section of the cable.
A standard cable maintenance ROV is equipped with water jets powered by on-board water pumps and these jets are used to excavate the shallow-sided depression. The maximum angle that the walls of the excavated depression may make with the horizontal plane depends upon the type of seabed material being excavated, for example sand or mud. Typically such an angle lies within the range 30° to 45°.
The excavation is performed until the relevant area of the cable is exposed at the bottom of the depression. The ROV rests at the perimeter of the depression and a manipulator arm attached to the ROV is then extended into the depression such that its remote end is positioned adjacent the cable.
A hydraulic cutter attached to the remote end of the manipulator arm is then used to sever the cable. A “gripper” having a lifting line attached to the surface vessel, is then attached to one severed end of the cable using the manipulator arm. The ROV then moves away and the cable is pulled to the surface using the line and a winch.
The ROV may also attach a pinger to the other severed end of the cable such that this may be easily located later and also raised to the surface.
Although this conventional method has proved successful for cables buried to around 1 meter, it has been found recently that some new fishing methods, employed mainly in the Far East, will now penetrate a soft seabed to a depth of around 2 metres. This of course places conventionally buried (1 meter depth) cables in danger of damage during fishing. One implemented solution to this problem is to bury the cables deeper within the seabed and the burial depth in some regions is now between 3 and 4 metres.
However, conventionally equipped ROVs are designed to locate and excavate cables buried only to a depth of around 1 meter, in addition to performing cutting operations and attaching recovery grippers. Such ROVs are also capable of reburying the cables to a similar depth of 1 meter.
Standard cable recovery ROVs have a typical power supply of about 200 horsepower which is used by the on-board systems such as upwardly directed thrusters (to increase the ROV's effective weight), manoeuvring thrusters, hydraulics to power a manipulator arm and water jet pumps.
Cables buried at greater depths cause a serious problem in that a much larger excavated depression is required such that the sides of the depression are maintained at the acceptable angle in order to prevent the sides caving in. Conventional ROVs have insufficient power to excavate such large depressions in a reasonable time and the design and construction of new more powerful ROVs capable of performing this task is extremely undesirable economically.